1. Field of the Invention
The present invention relates to a nonaqueous-electrolyte secondary battery incorporating a coil electrode formed by laminating elongated positive and negative electrodes through separators such that the outermost layer is the positive electrode.
2. Related Background Art
Secondary batteries for electronic apparatuses have been nickel-cadmium batteries or lead batteries. In recent years, the performance of the electronic apparatus has improved, the size has been reduced, and a portable structure has been developed. However, researchers continue to investigate secondary batteries with increased energy densities. There arises a problem in that the energy density of the nickel-cadmium battery and that of the lead battery cannot satisfactorily be raised because of low discharge voltages.
In recent years, a nonaqueous-electrolyte secondary battery has been developed and researched as a secondary battery expected to be capable of raising the discharge voltage and realizing small self-discharge and a long lifetime. The nonaqueous-electrolyte secondary battery has been employed in place of the nickel-cadmium battery and the lead battery. The nonaqueous-electrolyte secondary battery incorporates a negative electrode made of a material, such as a carbon material, which permits doping/dedoping of lithium ions; and a positive electrode made of a composite lithium oxide, such as composite lithium-cobalt oxide.
As described above, the nonaqueous-electrolyte secondary battery should to have a small self-discharge characteristic under a heavy load and a long lifetime. Therefore, the electrodes of the above-mentioned nonaqueous-electrolyte secondary battery have usually been formed into a coil electrode structure as shown in FIG. 1. As shown in FIG. 1, an elongated positive electrode 103 incorporates positive-electrode mix layers 102a and 102b formed by applying a positive-electrode mix to each of the two sides of a collector 101. An elongated negative electrode 106 similarly incorporates negative-electrode-mix layers 105a and 105b formed by applying a negative-electrode mix to each of the two sides of a collector 104. The positive and negative electrodes 103 and 106 are wound such that a separator 107 is interposed between the electrodes so that a coil electrode 108 is formed. In the foregoing case, an internal short circuit occurring when lithium is deposited during a charging operation must be prevented. Therefore, the width and length of the negative electrode 106 opposite to the positive electrode 103 usually are made to be longer than those of the positive electrode 103.
The above-mentioned coil electrode 108 is made so that the negative electrode 106 forms the innermost layer and the outermost layer above coil. The negative-electrode contains non-reacted active material which does not undergo charge/discharge at the end of the outermost layer of the negative electrode 106 and the innermost layer of the negative electrode. Therefore, the inside portion of the battery cannot effectively be used. As a result, the energy density of the battery is unsatisfactorily low.
To solve the above-mentioned problems, a technique has been disclosed in Japanese Patent Laid-Open No. 5-234620. As shown in FIG. 2, an elongated positive electrode 113 incorporates positive-electrode-mix layers 112a and 112b formed by applying a positive-electrode mix to each of the two sides of a collector 111. An elongated negative electrode 116 incorporates negative-electrode-mix layers 115a and 115b formed by applying a negative-electrode-mix to each of the two sides of a collector 114. The positive electrode 113 and the negative electrode 116 are wound such that a separator 117 is interposed so that a coil electrode 118 is formed. The outermost electrode, where charge/discharge of the coil electrode 118 is performed, is the positive electrode 113. Moreover, a portion of the positive electrode adjacent to the outermost end 113a of the positive electrode and/or a portion of the positive electrode adjacent to the innermost end 113b of the positive electrode is formed such that the positive-electrode-mix layer 112a is formed on one of the two main surfaces of the collector 111. Thus, the quantity of the non-reacted negative-electrode active material in the battery is reduced. The inside portion of the battery is more effectively used, thereby raising the energy density of the battery.
The above-mentioned coil electrode has the structure as shown in FIG. 2 such that the outermost end 116a of the negative electrode 116 is formed of only the collector 114. A negative-electrode lead 119 forms a projection on the upper surface of the collector 114. The positive electrode 113 has an outermost end 113a formed of only the collector 111. The negative electrode lead projection may pierce the separator 117 disposed between the negative electrode 116 and the positive electrode 113. The projection is undesirably brought into contact with the collector 111 of the positive electrode 113. A short circuit then takes place. As a result, with this structure, the percentage of defective batteries is undesirably increased and the reliability of the battery deteriorates.